In this experiment, you will investigate how much of the heat reaching a sensor from a hot plate comes from radiation and how much from convection. You plac...
Question 1 Report
In this experiment, you will investigate how much of the heat reaching a sensor from a hot plate comes from radiation and how much from convection. You places a temperature sensor 10 cm from a hot plate at 200 °C and records the reading. You then places a thin transparent glass screen between the plate and the sensor to block convection currents but allow radiation to pass through. You records the new sensor reading. You repeats the experiment with the glass screen replaced by a cardboard screen that blocks both convection and radiation. Fig. 28.1 shows the arrangement with the glass screen.
You records the following results. Without any screen the sensor reads 48 °C. With the glass screen it reads 39 °C. With the cardboard screen it reads 23 °C. The room temperature is 21 °C.
(a) Record the sensor reading with no screen. [1]
(b) Record the sensor reading with the glass screen. [1]
(c) Calculate the total temperature rise above room temperature with no screen. [1]
(d) Calculate the temperature rise due to radiation alone (glass screen reading minus room temperature). [1]
(e) Calculate the temperature rise due to convection alone. [1]
(f) State whether radiation or convection contributes more to the heating of the sensor. Use your calculations to justify your answer. [2]
(g) Explain why the cardboard screen gives a reading close to room temperature. [1]
(h) Describe one limitation of using a glass screen to separate radiation from convection. [1]
(i) State one precaution you takes to obtain reliable results. [1]
(j) State why you records the room temperature. [1]
Answer Details
(a) The sensor reading with no screen is 48 °C. [1]
This is the combined heating from both radiation and convection reaching the sensor from the hot plate.
(b) The sensor reading with the glass screen is 39 °C. [1]
The glass blocks convection currents but allows infrared radiation to pass through. The lower reading compared to part (a) shows that convection was contributing to the total heating.
(c) Total temperature rise above room temperature:
\[ \Delta T_{\text{total}} = 48 - 21 = 27\;^\circ\text{C} \] [1]
Subtracting room temperature isolates the heating caused by the hot plate alone.
(d) Temperature rise due to radiation alone:
\[ \Delta T_{\text{radiation}} = 39 - 21 = 18\;^\circ\text{C} \] [1]
With the glass screen blocking convection, the sensor only receives radiated infrared energy. The rise above room temperature is therefore due to radiation only.
(e) Temperature rise due to convection alone:
\[ \Delta T_{\text{convection}} = 27 - 18 = 9\;^\circ\text{C} \] [1]
The total rise (27 °C) equals the sum of radiation and convection contributions. Subtracting the radiation component (18 °C) gives the part carried by convection currents.
(f) Radiation contributes more to the heating of the sensor. [1] Radiation accounts for 18 °C of the temperature rise, while convection accounts for only 9 °C. Radiation delivers exactly twice as much heating as convection. [1]
At 200 °C the hot plate is a strong emitter of infrared radiation. The rate of thermal radiation increases rapidly with temperature, so at this temperature radiation dominates over convection at moderate distances.
(g) The cardboard screen blocks both radiation and convection, so almost no heat from the hot plate reaches the sensor. [1] Unlike glass, cardboard is opaque to infrared radiation and also acts as a physical barrier to air currents. The reading of 23 °C is close to room temperature (21 °C), with the small 2 °C difference likely due to minor heat conduction through the cardboard or air circulation around its edges.
(h) Glass absorbs some infrared radiation itself, so the measured radiation contribution may be lower than the true value. [1] Ordinary glass is not perfectly transparent to all infrared wavelengths. Longer-wavelength infrared is partially absorbed, meaning this method underestimates the radiation contribution and overestimates convection.
(i) Wait for the sensor reading to stabilise before recording it. [1] The sensor needs time to reach thermal equilibrium. Taking readings before the temperature has settled gives inconsistent values that do not represent the true steady-state temperature.
(j) Room temperature is recorded to use as a baseline for subtraction. [1] The temperature rise caused by the hot plate is found by subtracting room temperature from each sensor reading. Without this reference, you cannot distinguish between the heating effect of the hot plate and the background thermal energy already present in the room.
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